The present invention generally relates to an image forming apparatus in which an image is reproduced by plural developing devices arranged at the side of a photosensitive member.
Conventionally, as one example of an image forming apparatus as described above, there has been proposed an image forming apparatus in which a magnetic brush type two-component developer, composed of a toner and a carrier, held on the surface of a developing sleeve, is contacted with the surface of a photosensitive member to be electrostatically supplied to an electrostatic latent image on the member.
In this kind of apparatus, developer packaged density (P.D.) at a developing region confronting the sleeve with the member is defined by an equation which is EQU P.D.=Vd/Ds,
where Vd is an amount of the developer passed through the region per hour and Ds is a distance between the sleeve and the member. The density (P.D.) has a great effect on the quality of a reproduced image. That is, higher developer packaged density results in improved reproducibility RSI of the density of a reproduced solid image and/or a reproduced normal literal image and worse reproducibility RLI of the density of a line image, which indicates limited density of reproducibility of an original image with 50 .mu.m width when the line image is reproduced as a line. Conversely, lower packaged density results in worse reproducibility RSI of the density of the reproduced solid image and/or the reproduced normal literal image and improved reproducibility RLI of the density of the line image.
In other words, in the apparatus, the reproducibility RSI of the density of the solid image and/or the normal literal image conflicts with the reproducibility RLI of the density of the line image. That is, the former has priority of development over that of the latter to deteriorate the latter. The latter has priority of development over that of the former to deteriorate the former. Then, the quality of the reproduced image depends on the packaged density (P.D.).
Therefore, conventionally, it has been impossible for both of the reproducibility RSI and RLI of the density to simultaneously improve by only one developing device.
Then, in order to provide an image forming apparatus in which both the reproducibility RSI and RLI are improved, at least two developing devices are required. However, consumption amount of the toner in the developer in a case where the reproducibility RSI has priority over that of the reproducibility RLI is larger than that in a case where the reproducibility RLI has priority, over that of the reproducibility RSI, and thus, a large amount of toner are is replenished to the developing device with the priority for the reproducibility RSI. Furthermore, recently, there has been required to provide a miniature image forming apparatus, and then, the apparatus with both of the reproducibility RSI and RLI improved is needed to satisfy such a requirement.